"Everything is made up of water," Thales told us 2,600 years ago.
But he was speaking in the language of philosophers. The natural
scientist of our time says instead: "Everything is made of
elementary particles": the flowers in this hall, the bust of
Alfred Nobel, indeed every one of us - even our distinguished
Laureates! Elementary particles are nature's smallest building
blocks, the roots of everything.

In order to describe the building blocks of nature we must have
language, a theory. This year's Laureates in Physics, Gerardus 't
Hooft and Martinus Veltman, have made a decisive contribution to
this language. Their contributions concern electromagnetic and
weak interactions of the building blocks of matter.

Electromagnetism is the interaction which is responsible, for
instance, for the existence of atoms. The principal actor here is
the photon - light - because without light there would be no
electromagnetism. Weak interactions also have their own agents:
three particles that unfortunately have not been honored by
beautiful and dignified names but are simply called W-plus,
W-minus and Z. In spite of their dull names, these particles are
of paramount importance. Consider, for example, W-plus. We know
that the sun is like an oven. But who makes the fire there? The
particle W-plus, of course! Without weak interactions the sun
would not shine!

It turns out that the photon and the particles W and Z have a
common origin. Electromagnetism and weak interactions are thus
unified, and together they are called electroweak
interactions.

This year's Laureates had their predecessors – prominent
researchers who successively improved the description of these
interactions. But weak interactions appeared to act like loose
cannons. The calculations gave chaotic results – sometimes
excellent and occasionally completely absurd. Nonsensical results
appeared everywhere, as infinite probabilities and infinite
quantum corrections.

The theory of weak interactions was undoubtedly very sick. Today,
with hindsight, we see that it was in fact Veltman who indicated
the correct direction to be taken. His guiding star was the
concept of symmetry. The magic wand of symmetery converts a
little fragment here, and a little one there, into a complete
picture. The pattern that Veltman saw was that of a "non-abelian
gauge theory," also called "Yang-Mills" theory. Veltman embarked
on studies of weak interactions within the framework of these
theories and found encouraging results. A ray of light
illuminated weak interactions!

Although 't Hooft joined Veltman, as his Ph.D. student, a year or
so later, it was no easy task that awaited him. His contributions
to the solution of their common problem were simply dazzling. 't
Hooft and Veltman showed how the nasty infinities could be
harnessed and interpreted. First the theory had to be modified,
in order to be able to embark on calculations. This was done by
introducing, among other things, a number of ghosts - particles
that don't exist. These should, however, be well-mannered ghosts
which, at the final stage, would say goodbye and disappear. And
that is exactly what they do in 't Hooft and Veltman's methods.
Albert Einstein
taught us that we live in four dimensions – three spatial
dimensions and time. 't Hooft and Veltman tell us instead:
calculate as if the number of dimensions were slightly less than
four, four minus epsilon, i.e., 3.99999. This approach proved to
be highly effective. The nasty infinities became less
frightening. They could be collected, harnessed and
interpreted.

Although 't Hooft and Veltman did their – now prize-endowed
– work around 1970, it has taken a long time for us to
understand the extent of their ground-breaking efforts. We had to
wait for the results from an accelerator called the Large
Electron Positron (LEP) at the European Laboratory for Particle
Physics (CERN) on the outskirts of Geneva. This accelerator,
inaugurated in 1989 in the presence of – among others
– His Majesty, has set the world record in precision
measurements of electroweak interactions. 't Hooft and Veltman's
work has been a prerequisite for interpreting these results. The
LEP results showed that a sixth quark, the top quark, was needed
and that its mass could be determined even though there was not
then sufficient energy to produce it. In order for this
prediction to be confirmed we had to cross the Atlantic to the
Fermi National Laboratory near Chicago where, in 1995, the top
quark was discovered.

The theory of electroweak interactions predicts the existence of
an extremely interesting particle, called the Higgs particle,
whose mass can be determined with the help of 't Hooft and
Veltman's methods. Without the Higgs particle, says the theory,
we would all be massless and thus doomed to perpetual motion at
the speed of light. But then, we would not exist and thus not be
able to think (nor indeed – perish the thought! – to
enjoy this excellent evening). Will we discover the Higgs
particle? The future can be summarized with a single word:
Exciting!

Professor 't Hooft, Professor Veltman,

On behalf of the Royal Swedish Academy of Sciences I wish to
congratulate you for your groundbreaking work. I now ask you to
step forward to receive your Nobel Prizes from the hands of His
Majesty the King.